References handling: Fix bugs introduced by 7d88538d

* source and group attributes were being added as HTML attrs.
  on the ol-node which is invalid.  Plus, after refererences
  generation, source attr was being deleted which causes crashers
  in production if references output is reused from cache.

* Moved both attributes to data.parsoid

* Removed group="" from parserTests that I erroneously added
  in that previous commit.

* Minor cleanup in DOMPostProcessor.

Change-Id: I85f4ad0a311d653d7ba3e010c7a5db028b643e9f